Method and apparatus for the detecting op conducting
bodies and massive and disseminated ore bodies
utilizing electromagnetic waveforms
exhibiting abrupt discontinuities



Nov. 16, 1965 A. R. BARRINGER Re. 25,908

METHOD AND APPARATUS FOR THE DETECTING OF CONDUCTING BODIES AND MASSIVEAND DISSEMINATED ORE BODIES UTILIZING ELECTROMAGNETIC WAVEFORMSEXHIBITING ABRUPT DISCONTINUITIES Original Filed Jan. 5. 1960 9Sheets-Sheet 1 Inventor ANTHONY R BA RR/NGER NOV. 16, 1965 BARRlNGER Re.25,908

METHOD AND APPARATUS FOR THE DETECTING 0F CONDUCTING BODIES AND NAssIvEAND DIssEMINATED ORE BODIES UTILIZING ELECTROMAGNETIC WAVEFORMSEXHIBITING ABRUPT DISCONTINUITIES Inventor ANTHONY R, BARRINGER Nov. 16,1965 A. R. BARRINGER Re. 25,908

METHOD AND APPARATUS FOR THE DETECTING OF CONDUCTING BODIES AND MASSIVEAND DISSEMINATED ORE BODIES UTILIZING ELECTROMAGNETIC WAVEFORMSEXHIBITING ABRUPT DISCONTINUITIES 9 Sheets-Sheet 3 Original Filed Jan.5, 1960 FIGS I0 SECONDS Inventor ANTHONY R. BARRINGER b 4150 s00MICROSECONDS Nov. 16, 1965 A. R. BARRINGER Re. 25,908

METHOD AND APPARATUS FOR THE} DETECTING OF CONDUCTING BODIES AND MASSIVEAND DISSEMINATED ORE BODIES UTILIZING ELECTROMAGNETIC WAVEFORMSEXHIBI'IING ABRUPT DISCONTINUITIES Original Filed Jan. 5, 1960 9Sheets-sheet 4 i MICROSECONDS b 450 000 MICROSECONDS M/CROSECONDS FIG.73 FIG. 14 FIG. 75

Inventor ANTHONY R4 BARRINGER MHW Nov. 16, 1965 A. R. BARRINGER Re.25,908

METHOD AND APPARATUS FOR THE DETECTING OF CONDUCTING BODIES AND MASSIVEAND DISSEMINATED ORE BODIES UTILIZING ELECTROMAGNETIC WAVEFORMSEXHIBITING ABRUPT DISCONTINUITIES Original Filed Jan. 5, 1960 9Sheets-Sheet 6 FIG. 17

ANTHONY R- BARRINGER Re. 25,908 ATUS FOR THE DETECTING OF CONDUCTINGSSIVE AND DISSEMINATED ORE BODIES UTILIZING ELECTROMAGNETIC WAVEFORMSEXHIBITING ABRUPT DISCONTINUITIES 5, 1960 Nov. 16, 1965 A. R. BARRINGERMETHOD AND APPAR BODIES AND MA 9 Sheets-Sheet 7 Original Filed Jan.

FIG. 17a

Inventor ANTHONY R. BARRINGER Nov. 16, 1965 A. R. BARRINGER METHOD ANDAPPARATUS FOR THE DETECTING OF CONDUCTING BODIES AND MASSIVE ANDDISSEMINATED ORE BODIES UTILIZING ELECTROMAGNETIC WAVEFORIVIS EXHIBITINGABRUPT DISCONTINUI'IIES Original Filed Jan. 5, 1960 9 Sheets-Sheet 8FIG. 18

Inventor ANTHONY R. BARRINGER FIG. 19

Nov. 16, 1965 A. R. BARRINGER Re. 25,908

METHOD AND APPARATUS FOR THE DETECTING OF CONDUCTING BODIES AND MASSIVEAND DISSEMINATED ORE BODIES UTILIZING ELECTROMAGNETIC WAVEFORMSEXHIBITING ABRUPT DISCONTINUITIES Original Filed n- 5. 1960 9Sheets-Sheet 9 CHANNEL CHANNEL CHANNEL CHANNEL ALTIME TER 5 CHANNEL MILEGRID LINES FIG. 20

Inventor AN rHoNY R BARRINGER United States Patent 0' This inventionrelates to a method and apparatus for the remote detection of conductingbodies.

Present day airborne electromagnetic geophysical systerns normallyoperate by continuously transmitting one or more audio frequencies fromcoils mounted on aircraft, and detecting the secondary response fromconductors in the ground by using receiving coils mounted on theaircraft or towed behind in a bird. Since the wave forms transmitted arecontinuous, it is necessary to detect the secondary field in thepresence of the primary field which involves elimination of the primaryfield from the receiving system. This is generally achieved by one oftwo methods: Firstly by holding the receiving and transmitting coils insome fixed orientation relationship and electronically balancing out thecomponent of the primary field detected in the receiving coil, orsecondly by eliminating all components which are in phase with thetransmitted field and detecting only that component in the receivingcoil which may be resolved into quadrature phase relationship with thetransmitted field. The re ceiving coils may be mounted in the plane ortowed behind in a bird. If the receiver is mounted in the aircraft,

the sensitivity of the receiving coil to vibration and attendantvariations in the pick-up of the transmitted field is acute since minutemovements of the receiving coil will strongly modulate the coupling withthe transmitted field. Where quadrature components only are measured, I

a high noise is introduced by close coupling of the receiver coil in theplane with quadrature components of the induced currents in the metalaircraft frame. If the receiving coil is towed behind the plane in abird,

limitations on sensitivity are determined by the inevitable I variationsin position of the bird in relation to the aircraft and the geometry ofthe transmitted field. Only the pure quadrature system after suitablecompensation for the aircraft frame currents is virtually free frommisorientation noise when a towed bird is used and this system suffersthe disadvantage that considerable informa' tion is lost when onlyquadrature measurements are made. Some attempts have been made toovercome the latter disadvantage by the use of two frequenciessimultaneously, the quadrature responses at two frequencies beingequated as roughly the same as the use of in-phase and quadraturemeasurements at one frequency. This technique however is not Whollysatisfactory because different currents paths are energized at differentfrequencies in naturally occurring conductors in the ground andconsequently the interpretation of the results is considerably increasedin ambiguity.

Another limitation of quadrature systems together with all existingairborne electromagnetic systems is that in practice the receiving coilmust be used in maximum coupling with the transmitted field since slightchanges in orientation in the position produce a much smaller per- Re.25,908 Reissued Nov. 16, 1965 centage change in coupling (a function ofthe cosine of the displacement angle) than in other positions,particularly as compared with the null coupling position (a function ofthe sine of the displacement angle). Consequently, existing airborneelectromagnetic systems almost exclusively measure the one component ofthe secondary field which is resolved along the direction of the axis ofthe receiving coil.

In contrast to the above, it has been found that it is possible with thepresent invention in which transient respouses are used, to measure thesecondary field of a conductor during a period when the primary field iseither absent or not time varying, regardless of the orientation of thereceiving coil.

Using this technique, it becomes possible to measure secondary fieldswithout any noise levels due to the interference of the primary fieldregardless of variations in coupling between the receiving andtransmitting coils. Furthermore, all three components of the secondaryfield can be measured using three mutually perpendicular receivingcoils. This added information is of value for the following reasons.Firstly, universal coupling is maintained with conductors in the groundregardless of flight direction. In other systems conductors may bemissed if they are accidentally crossed at an angle where the receivingcoil is in null coupling with the ground conductor. Secondly,information can be deduced from the three component responses on the dipof the ground conductor enabling positive discrimination betweenconductive overburden such as swamp and dipping conductors of possibleeconomic value.

A further very important advantage of the invention as compared withother airborne electromagnetic systems, particularly of the quadratureor out of phase type, is that transient analysis gives information onthe inductive response equivalent to the use of a large number offrequencies. When two conductors are present, such as is commonly thecase when a dipping conductor underlies conductive swamp or clay, thetrue conductivity of the underlying conductor can be estimated withoutmodification by the overlying conductor.

The invention also has application in the domain of radio frequenciessuch as microwaves where it is possible to make measurements pertainingto the amplitude and conductivity on reflected radar type wave forms, orwave forms showing abrupt terminations, without reference to the phaseof the transmitted signal and the travel time to and from the reflectedsurface.

Accordingly, it is amongst the objects of the invention to provide amethod and apparatus for the measurement of the secondary field of aconductor whilst the primary field is eliminated.

It is further amongst the objects of this invention to provide a methodand apparatus for the measurement of the secondary field of a conductorregardless of the orientation of the receiving coil.

It is also amongst the objects of this invention to provide a method andapparatus for the measurement of the secondary field of a conductorutilizing a discontinuous electromagnetic wave form to energize theconductor.

Further objects and advantages will become apparent from a considerationof the following description having reference to the drawings in which:

FIGURE 1 shows a single current pulse wave form of the type circulatingin the transmitting loop;

FIGURE 2 shows a partially cut away perspective view of a threereceiving coil bird according to the invention;

FIGURE 3 shows a perspective view of the suspension means for the bird;

FIGURE 4 shows a block diagram of one channel of the receiving system ofthe invention;

FIGURE 5 shows a received wave form of a primary pulse followed by atransient field response from a conductor of high conductivity;

FIGURE 6 shows a similar received wave form followed by a transientfield response from a conductor of low conductivity;

FIGURE 7 shows a wave form of the signal received from the tunedamplifiers and the position of secondary detection gating;

FIGURE 8 shows a typical recorded response on a single channel picked upduring flight traverse across a con ductor;

FIGURE 9 shows a simple transient field response from a single conductorillustrating the positioning of samples which may be taken by the firstdetector gates;

FIGURE 10 shows a typical orientation of an ore body of goodconductivity underlying swamp of poor conductivity;

FIGURE 11 shows a complex transient wave form arising from twoconductors, one of high conductivity and the other of low conductivitysimilar to the bodies illustrated in FIGURE 10, broken lines illustratethe two component transients which make up the resultant complextransient;

FIGURE 12 shows a complex transient wave form illustrating positioningof three first detector gate samplings;

FIGURE 13 shows typical triple first detector gate samplings of thetransient wave response received from the vertical transverse coil;

FIGURE 14 shows typical single first detector gate sampling of thetransient wave response received by the horizontal coil;

FIGURE 15 shows typical single first detector gate sampling of thetransient wave response received by the vertical longitudinal coil;

FIGURE 16 shows a block diagram of an electromagnetic detecting systemof the invention utilizing multiple channels;

FIGURES 17 and 17a show in two parts a schematic wiring diagram givingdetails of a pulse generator transmitter circuit suitable for use withthe present invention; and

FIGURE 18 shows a schematic wiring diagram of a pre-amplifier for thesignals received in the receiving coils suitable for use with thepresent invention;

FIGURE 19 shows a schematic wiring diagram of one channel of thereceiving portion of the receiver in a form suitable for use with thepresent invention;

FIGURE 20 shows an example of the type of recording obtained accordingto the present invention.

Referring now to the drawings, current pulses of the form shown inFIGURE 1 are generated by a pulse generator adapted to produce 80 pulsesper second normally into a loop transmitter thereby radiating a pulsedelectromagnetic field. Primary current pulse is seen as a differentiatedwave form when detected as voltage in a receiving coil.

A circuit diagram for a transmitter suitable for use with the instantinvention is shown in FIGURE 17. The loop of the transmitter may bemounted on an aircraft (not shown).

The bird 10 shown in FIGURES 2 and 3 is adapted to be towed by cable 11at a distance of about 500 feet from an aircraft (not shown). Thesuspension of bird 10 is shown in more detail in FIGURE 3. The cable 11which contains the electrical connections and tow cable is joined to anelastic shock cord 12 by suitable means at 13. The elastic shock cord 12forms a loop which is joined at its ends to each side of the bird as at14 by suitable means such as a bolt arrangement. of a heavy elastic cordin a woven casing and is designed to isolate cable vibration from thebird. The bird 10 contains three inductive detectors in the form ofthree ferrite cored coils 15, 16 and 17 wound to a resonant frequency of12 kc. Coil 15 is the vertical longitudinal coil respon- The shock cord12 is formed sive to the horizontal component of fields resolved in thedirection of flight. Coil 16 is the horizontal coil responsive to thevertical component of fields and coil 17 is the vertical transverse coilresponsive to the horizontal component of fields at right angles to thedirection of flight. In actual practice, coils 15 and 16 may consist ofa plurality of coils wound on short ferrite rods and joined together inseries in ladder fashion. In this way, a suitable saving of space may beeffected within the bird 10. The coils may be coated in silicone rubberwhich cures at room temperature and must be very carefully acousticallyisolated with felt and foam rubber to minimize microphonics. Each of thecoils 15, 16 and 17 is independently critically damped by means ofadjustable potentiometers 18, 19 and 20. The inside of the bird 1!) isprovided with Faraday shielding 21 further to reduce interference, theFaraday shielding being grounded through cable 11.

Three pre-amplifiers 22, 23 and 24 having a frequency response of 5c.p.s.2() kc. are provided for the coils 15, 16 and 17. The amplifiersshould have good transient re-' sponse and noise level not more than 10microvolts peak to peak at the input.

The concept of the separation of transients involves" time gating in thereceiver system of the type shown in FIGURE 4. In the case ofaudio-frequencies where conductors are being detected at relativelyshort range using induction fields, the return signal is delayed only aninsignificant portion of time with respect to the overall dimensions ofthe transmitted wave form. The gate may therefore be initiated bytriggering circuits associated directly with the transmitter. When radiofrequencies are involved in radio type applications and wheremeasurements are being made on a return pulse which may be delayedsubstantially in time with respect to the outgoing pulse, it becomesnecessary to trigger the gates by the leading edge of the return pulse.

Referring now to FIGURE 4, a block diagram of the circuitry of a singlechannel is shown. (An example of the wiring diagram of the receivingportion of the circuitry shown in FIGURE 4 is given in FIGURE 19.) Theuse of multiple. channel detection is considered more fully hereafter.Signals from a coil such as 15 in the bird 10 pass through thecorresponding preamplifier 22. (By way of an example a schematic wiringdiagram for a pro-amplifier suitable for use with the present inventionis shown in FIGURE 18.) A high pass filter 25 having a low cut-offfrequency of ten c.p.s. is interposed between the amplifier 22 and firstdetector gate 26. The high pass filter 25 passes the received pulse andsecondary field transient without distortion. lt filters out the lowfrequency signals generated by the movement of the bird 10 and itsreceiving coils in the earths magnetic field.

The first detector gate 26 is conveniently adjustable between 20microseconds and 1 millisecond in width with normal operation of 100microseconds. It is arranged to take a sample of the signal detectedimmediately following the transmitted pulse. Thus, if any secondarytransient field is present, it will be detected.

First and second tuned amplifiers 27 and 28 are two amplifiers arrangedin series and tuned to the same frequency as the pulse repetition rate.Pulses are received at the tuned frequency of the amplifiers from thefirst detector gate. Thus, if the first detector gate is sampling randomnoise, it will pass signals of randomly varying polarity and amplitudewhich will tend to integrate out in the tuned amplifiers 27 and 28. If,however, the first detector gate is sampling a transient secondaryfield, as shown by the wave form of FIGURE 5 the signal will cohere andgenerate an c.p.s. signal in the tuned amplifiers.

The second detector gate 29 samples the output of the tuned amplifiersat a repetition rate locked to the transmitted pulse rate, i.e., to thetuned frequency of the amplifiers. A11 illustration of the signal andgating is shown in FIGURE 7. The second detector gate is designed to beadjustable in width from microseconds to 10 milliseconds. It is alsoadjustable in phase so that samples are taken at the peaks of the sinewave from the tuned amplifiers as shown.

The integrator consists of a passive low pass filter network arranged toconvert the pulse from the second detector gate into a slowlyfluctuating DC. signal. The time constant of the low pass filter orintegrator 30 is conveniently adjustable between 1 and 4 seconds. Theintegrator or low pass filter 30 has the function of integratingcoherent signals over the period of the time constant of the networkwhilst tending to integrate out and destroy noise fluctuations of randompolarity which manage to pass the tuned amplifier system. The longer thetime constant the narrower the effective bandwidth of the system and thegreater the reduction in noise.

Normally, a two second time constant would be used for traversing at 500feet above ground level. FIGURE 8 shows a typical anomaly detected at500 feet with the plane flying at 100 miles per hour from which it canbe seen that the length of the time constant of the integrator may beincreased to a maximum of 4 seconds to smooth out the signal and reducenoise variations, but if the time constant is made too long (e.g. 10seconds) the shape of the anomalies produced by conductors below theplane will be seriously distorted.

The recorder 31 is most conveniently of the high sensitivity mirrorgalvauometer type. A typical such recorder which would be suitable foruse with the present invention is the Visicorder (a trademark ofHoneywell Controls Ltd.) model 1012, manufactured by Honeywell ControlsLtd. This has the advantage of eliminating D.C. amplifiers from thesystem with the concomitant simplification of the system and eliminationof the risk of trouble occasioned by D.C. drift.

Moreover, the mirror galvanometer permits the use of passive rather thanactive integrators. It also utilizes high intensity ultra violet lampsand special paper thus permitting direct recording without the need forchemical developers.

The above description relates to the receiving of pulses from a singlecoil. It will be appreciated that separate channels are used to recordthe responses of each coil. Moreover, if more than one primary gating isto be sampled from each coil, it will require a separate channel toeffect this. A schematic block diagram showing multiple channels isillustrated in FIGURE 16.

Thus it can be seen that by utilizing three coils such as 15, 16 and 17,it is possible to measure all the components of the secondary field.Flat conductors give a different ratio of response in the vertical andhorizontal coils and a different relative positioning of the peakresponse as compared with inclined conductors. Thus by measuring all thecomponents of the secondary field it is possible to estimate anorientation of the conductor in the ground and recognize the conductivebottoms of lakes and swamps and differentiate them readily from dippingconductors.

A more complicated situation arises in the case of conductors disposedas shown in FIGURE 10, Le. an ore body of good conductivity underlying abody of poor conductivity such as a swamp. A consideration of the mannerof distinguishing such a situation according to the present inventionwill be more readily understood from the following discussion of theprinciple of measurement of the conductivity of conductors.

The measurement of the conductivity of conductors is based upon theestimation of the time constant of the decaying transient produced whenthe conductor is energized with a pulse. Normally an ore body consistingof base metal values occurring in a massive sulphide deposit may beconsidered as a conductive sheet in which circulating current will beinduced in the presence of a time varying electromagnetic field. Ingeneral a current pulse of the type shown in FIGURE 1 circulating in atransmitting loop adjacent to a conductive sheet will induce a similarcurrent pulse in the conductive sheet. This pulse will tend to circulatein the loop formed by the periphery of the sheet. Transient effects atthe termination of the pulse will be similar to the transient decay ofcurrent in a coil or a loop of wire. The current will decayexponentially in the following manner.

Let the current after a time t after termination of the pulse:i(t).

Let the voltage of the termination of the pulse v.

Let the resistance of the current loop R.

Then

:33 Ve L U) When the exponent equals 1 and the current has fallen to36.8% of its value at the moment of final collapse of the energizingfield.

The value of is defined as the time constant T of the circuit.

In a conducting sheet L is a function of the size of the sheet and thecurrent loop which circulates around its periphery. R is a function ofthe specific resistivity of the material of the sheet and its thickness.As an approximation for a given conductive material and a giventhickness of sheet. the time constant L/R is constant since L and R bothvary lineally with the distance around the circulating current loop.Hence a measurement of the time constant gives a measure of theconductivity characteristics of a sheet which aids in the recognition ofvaried ore deposits. For a simple sheet the time constant is directlyrelated to the measurement of phase shift made with a continuous wavesystem.

In the case where disseminated sulphide particles are present in anaturally occurring conducting mass or sheet, the transient effectscannot be approximated solely by considering them in terms of inductanceand resistance. Polarization effects and capacitance effects occur atthe interface of the conductive sulphide particles with the enclosingrock material. These effects are recognized by exceptionally long decayperiods and are of considerable diagnostic value. The evaluation ofpolarization and capacitance effects is virtually impossible with asingle frequency continuous wave approach.

The time constants of conductors are measured in principle by using twochannels in which samples A and B are taken with a time separationoccurring between them (see FIGURE 9). Since the transient has anexponential form the ratio of A/B is the same for all parts of the curvehaving a fixed time separation and the ratio of A/B is related to thetime constant.

Let the initial voltage :V.

Let the voltage after time t A.

Let the voltage after time t :B.

Let Tc time constant of transient.

Then in the general case for an exponential decay -t Voltage after timet=Ve Therefore :3 A=Ve :53 B Ve Tc The following are some typical valuesof time constant for various ratios of A/B assuming a separation of 100microseconds between A and B.

When

3 123 T0 500 microseconds The time constant for naturally occurringground conductors varies considerably and the following may be taken byway of a guide:

To 80 microseconds Time Constant Classification of Conductor 20-100Microseconds Poor. eg. conductive swamp, lake Wtlltl', etc. 100400l\licroscconds Medium, cg. weakly mincrnlizcd shear zones, lake bottomsilts, conductive clays.

201) Microseconds-l Millisccond Good, cg. massive sulphide ore bodies,barren sulphides, graphite.

Over 1 illisecond Polarizing conductor. Disseminated sulphide depositscxliibiting capacity and induced polarization eilccts.

From the foregoing it will be understood that the term conductor as usedin this specification includes both electrical conductors and polarizingconductors.

However, it will be appreciated that all transient wave forms will notbe of a simple nature. The presence of disseminated sulphide particlesor the presence of two conductors disposed as shown in FIGURE 10 willcause variations in the transient wave form, as illustrated by thetransient wave form shown in FIGURE 11.

In the case of complex transients of the form shown in FIGURE 11, itwill be seen from the figure that the left-hand or steep portion of thecurve represents a short time constant arising from a poorly conductiveoverburden. The lower right-hand part of the curve shows a long timeconstant arising from an underlying good conductor.

In order to measure complex transients, it is necessary to take threesamples, A, B and C. (See FIGURE 12.) It will be apparent that for asimple exponential transient or Ao=i3 or =1 (See FIGURE 9 For a complextransient involving a mixture of exponential curves AC/B is equal to aratio greater than 1. Thus the ratio AC/B can be used to determine thedegree of complexity of the transient. The ratio of A:B and B10 and ACzBmay be obtained electronically.

The long tail of a transient response from a good conductor underlying apoor conductor as shown in FIGURE 10 is a major feature of theinvention. A relatively undistorted estimate of conductivity of aconductor masked (ill by swamp and the like may be made. In present daycontinuous wave systems using only one frequency for in phase andquadrature measurement or two frequencies for quadrature measurementonly, conductivity estimates based on phase shifts are severelydistorted by the presence of conductive overburden since mixed phasescannot be separated.

In the present invention the parameters of a complex transient are mostconveniently measured on the vertical transverse coil. Three signalsamples A, B and C are taken as shown in FIGURE 13 and fed to respectivechannels designated 1, 2 and 3. By the use of suitable delays the ratiosof signal B in channel 2 to signal C in channel 3 can be used to make arapid determination of the true conductivity of masked conductors (seeFIG. 12). If channel 1 is set immediately to follow the pulse andchannel 2 is delayed 400 microseconds and channel 3 is delayed a totalof 800 microseconds, channel 2 and channel 3 will not receive any signalfrom conductors of low conductivity due to the short time constant ofthe transient response received therefrom (see column 7, supra)conductive swamp, clay or lake bottoms. They will however detect goodconductors masked thereby and also disseminated sulphide zones of lowconductivity which exhibit long transients clue to polarization and selfcapacity effects. Disseminated sulphides can form commercial ore bodiesand the invention gives a new approach to their discrimination from thefar more numerous other types of low conductivity conductors. Signalsamples D and E shown in FIGURES 14 and 15 are derived from thehorizontal and vertical longitudinal coils respectively and are fed torespective channels 4 and 5. They are not used to measure conductivityparameters of the conductors, but solely to assist in the determinationof the orientation of the conductors both in respect to the horizontaland in respect to the direction of the flight line traverse.

A typical illustration of the type of recording obtained using all livechannels is illustrated in FIGURE 20. As previously explained, thedeviations in the recordings on channels 1, 2 and 3 are utilized todetermine the nature of the conductor whilst the deviations in therecordings on channels 4 and 5 give additional information as to theorientation of the conductor.

The complete system may also include a Doppler radar navigation systemfor facilitating accurate survey traversing in parallel traverse spaced:1 quarter of a mile apart. Marker pulses are provided every one tenthof a mile from the Doppler radar and these are recorded on thegeophysical recorder facilitating computations on depth and dip based onthe profiles recorded.

A radar altimeter is used to provide a continuous recording alongsidethe geophysical profiles of ground clearance. This enables correctionfor flying height to be carried out. Normal survey flying height is 500feet.

A continuous strip 35 mm. camera is used to photograph all the groundsurveyed and fiducial numbers provided by an output from the Dopplerradar are simultaneously recorded on the camera and on the geophysicalrecorder. This feature enables geophysical anomalies to be located on amap prepared from aerial photographic mosaics with an accuracy of betterthan plus or minus 200 feet.

Referring now to the circuit diagrams shown in FIG- URES 17, 17a, 18 and19, the pulse transmitter is shown in FIGURES 17 and 17a. It consistsessentially of a thyratron pulse generator in which the inductive partof the pulse forming network consists of an electromagnetic radiatingloop or magnetic dipole.

Two blocking oscillators indicated generally by references 32 and 33 aretriggered from an external low level trigger source (not shown) providedat terminals 34 and 35. Blocking oscillators 32 and 33 in turn developrespective output pulses which are then amplified by respective pulseamplifiers indicated generally by references 36 and 37, each of whichcomprises a small thyratron,

9 36a and 37a, and a clipping diode 36b and 37b. The amplified pulsesare then communicated to the large thyratrons 38 and 39 and providetriggering pulses therefor which in turn will permit discharge ofcapacitor bank 40 through radiating loop 41, thus providing the desiredpulse therein of high ampltiude and of short duration.

Capacitor bank 40 is then re-charged by a DC. power supply comprisingthe power transformer 42, the input side of which in connected through asuitable variable autotransformer 74 to an AC. source (not shown). Theoutlet side of transformer 42 is fed to the full wave rectifier 75 whichin turn is provided with two filter chokes 76 and connected to thefilter capacitor bank 77 and re-charges the same during the zero portionof the pulse transmitter cycle.

It will also be noted that blocking oscillators 32 and 33, pulseamplifiers 36 and 37 and thyratrons 38 and 39 are provided with DC.power from an auxiliary D.C. source generally indicated by reference 78.Source 78 is of generally conventional design, incorporating a timedelay relay 79 which remains open until operating temperature isreached, and further incorporating the ready light 80 indicating thatrelay 79 is closed, and warning light 81 indicating that the circuit ison.

In addition to the foregoing the instant pulse transmitter incorporatesfurther safety devices to minimize breakdown hazards.

Thus relay 82 is provided, including contact 83 which is designed tooperate only when relay 79 has closed and ready light 80 has come onindicating that the transmitter is ready for operation, and will thenclose on momentary depression of the on switch and open on momentarydepression of the off" switch, thus making and breaking the supply ofpower to autotransformer 74.

Interlock 84 is provided, consisting essentially of cut out relay 85which is designed to remain normally closed during operation and to openwhen thyratrons 38 and 39 break down and remain conductive.

Similarly interlock 86 is provided consisting essentially of cut outrelay 87 which is also designed to remain normally closed duringoperation and to open when thyratrons 36a and 37a break down and remainconductive. Disconnect switch 88 controls the supply of power to powersupply 78.

The transmitter described above is designed to give a minimum averagepower dissipation in the loop of 2 kv.-a.

The pre-amplifier shown in FIG. 18 is a low noise type withapproximately 70 db of gain, the principal components of which are thetransistors 43, 44 and 45 which are type 2N207B, low noise audiofrequency PNP transisters and transistors 46 and 47 which are type 2N44audio frequency PNP medium power transistors. The amplifier is class Athroughout and has a frequency response of cycles to kilocycles plus orminum 2 db. The amplifier of FIGURE l8 is housed in the bird.

The circuitry of a complete channel less the coil and pre-amplifier isillustrated in FIGURE 19. The signals from the bird preamplifier are fedinto a high pass filter 48 of conventional design with a cut offfrequency of 5 cycles per second. This filter is designed to reject lowfrequency signals generated by slow movements of the bird detector coilsin the earths magnetic field. The signal passes from the filter into thefirst detector 49. The first detector is a sampling gate consistingessentially of two transistor switches 50 and 51 in cascade relationshipwith one another. The transistors 50 and 51 are type 2N5l8, which arelow noise fast switching PNP transistors each of which switches thesignal to ground, two being used in cascade to ensure optimum effect. Aresistor 52 provides isolation from the reactance circuitry of thefilter 48.

A balancing control 53 is provided for adjusting the amount of voltagepedestal produced by the switch and is a special feature of thecircuitry used for zeroing the DC. output of the system appearing afterthe second detector and integrator. The gate pulse which controls theswitching action is fed in at 54, its level being regulated by the Zenerdiode 55. A DC. bias is fed in at 56 to make the transistors 50 and 51normally conduct, the transistors then being switched off by the gatepulse applied at 54. The detector or sampling gate 49 is designed tohave greater than 40 db rejection in its shorted condition. The signalis passed from the first detector 49 to the first tuned amplifier 57.Transistor 58 is a 2N44 PNP type and acts as a bufier between the outputof the first detector and transistor 59 which is also a 2N44 and acts asa tuned amplifier in conjunction with the high Q toroidal inductor 60.The tuned amplifier is tuned to the transmitted pulse repetition rate.Transistor 61 is a 2N44 type which acts as a further buffer between thefirst tuned amplifier 57 and the second tuned amplifier 62. Transistor63 is a type 2N44 and together with inductor 64 forms a further tunedamplifier tuned to the transmitted pulse repetition rate. Transistor 65is a type 2N44 transistor used as an audio amplifier of high dynamicrange and feeds into the second detector switch 66. This switch is asingle section switch similar in type to each of the switches used inthe first detector 49. It functions as a gated sampler of the sine waveoutput from the tuned amplifiers and as such is in the nature of. aphase detector. The switch features a wide dynamic range to cope withthe considerable variations in amplitude which in practice appear in thesystem. The second detector feeds into a low pass filter 67 which actsas a simple integrator of the pulses delivered by the second detector66. The values of the capacitor 68 and resistor 69 are chosen to providesuitable time constants and an additional capacitor 70 may be switchedin to provide an alternative longer time constant. Typical values oftime constants chosen are two seconds and four seconds. The Zener diode71 provides a regulated bias voltage derived from voltage applied at 72,said voltage being used to maintain correct polarity on the electrolyticcapacitors used for convenience in the integrator. The coil of themirror galvanometer used in the recorder is shown at 73. As notedpreviously, the pedestal of the first detector gate may be used tocontrol the DC. output of the integrator to the mirror galvanometer. Thefirst detector gate is locked in time relationship to the transmittedpulse and the pedestal output therefore energizes the tuned amplifiersat their resonant frequency and in coherent phase relationship. Thebalance control 53 adjusts the gate pedestal and will therefore exercisecontrol on the amplitude of the sine wave signal generated by the tunedamplifiers. This sine wave output of the tuned amplifiers issubsequently detected by the second detector and fed to the integrator67. Hence by adjusting the pedestal using the balance control 53, theoutput of the integrator may be varied and the mirror galvanometerthereby conveniently zeroed.

The above references to component types are according to Americanmanufacturers standard classification.

The invention has been described having particular reference to the useof a transmitted signal in the form of a half sine wave pulse generatedat a repetition rate of per second. The half sine wave shape has beenchosen because it is a relatively simple and efficient discontinuouswave form to generate. Using this shape a pulse length of not less than1 millisecond is required to obtain efficient energization of goodconductors in the ground which typically exhibit time constants in theregion of 500 microseconds. Shorter pulses will reach peak amplitude toorapidly to energize said ground conductors fully. On the other hand longpulses greater than 3 milliseconds in length are excessively wasteful ofpower and little or no additional energy is induced into the groundconductors by prolonging the pulses unduly. Furihermore if a half sinewave of greater than 3 milliseconds in length is used the slow decay ofthe pulse minimizes transient effects.

It is desirable to have at least 1 millisecond following waveformdiscontinuities such as the termination of a pulse in which to measuretransient effects. Thus if half sine pulses are used a maximum practicalrepetition rate is 500 per second using 1 millisecond pulses and 1millisecond intervals between pulses. Using the circuitry described, thelower limit is set at about 40' per second since the inductively tunedamplifiers which have been used become very ineflicient below thisfrequency. However a. minor modification to the circuitry in whichconventional R.C. tuned amplifiers are substituted for inductively tunedamplifiers enables pulse repetition rates as low as 1 per second tobecome perfectly practical. In order to achieve the same efficiency andsignal to noise ratio at differing repetition rates it is merelynecessary to keep the average power dissipated in the radiating loop thesame. Thus at low repetition rates much higher peak powers are requiredand vice versa. The lower rate of repetition is ultimately set byconsideration of the rate of traverse of the survey aircraft over theground; for example, the maximum desirable interval between measurementsat 100 mph. would be about 1 per second.

Wave shapes other than half sine pulses may be employed without anydeparture in principle from the procedures outlined. Suitable waveshapes include sawtooth forms, square pulses and any wave formexhibiting discontinuities or departure from simple harmonic functionswhen followed by periods in which the primary field is not time varying.Modifications to the transmitter can be carried out by one skilled inthe art to change the transmitted shape to alternative configurations.If discontinuities are applied at regular periodic intervals thencircuitry identical to that described here can be used in the receivingsystem, the tuned amplifiers of the system being tuned to theperiodicity of the transmitted discontinuities, and the gates beingtriggered in fixed time relationships to the primary fielddiscontinuities.

Discontinuities may also be triggered at random intervals providing thereceiving system is triggered in locked time relationship to thetransmitted system. The only modification required to the receivingsystem is the substitution of wideband, wide dynamic range, A.C.amplifiers for the tuned amplifiers. By way of example, triggering maybe accomplished by using a zero-crossing detector on one of thereceiving coils to provide trigger pulses only when zero volts exist inthe detector coils. This types of technique can be used as analternative to a high pass filter as a means of eliminating lowfrequency noise due to low frequency movements of the bird and itsreceiving coils in the earths magnetic field.

It will be appreciated to those skilled in the art that variations andmodifications may be made without departing from the scope of theinvention as defined in the following claims.

What I claim is:

l. The method of detecting conducting bodies, and comprising: initiatingprimary waveforms exhibiting abrupt discontinuities at predeterminedintervals from a moving source; radiating said primary waveforms towardsareas in which conducting bodies are to be detected, thereby to inducetransient secondary electromagnetic Waveforms emanating from saidconducting bodies; moving receiving means at a velocity substantiallyequal to that of the moving source; receiving said primary waveforms andsecondary reradiated transient effects emanating from conducting bodiesenergized by said primary waveforms and discerning said received signalsin three component directions mutually at right angles. one of saidcomponents being horizontal and substantially at right angles to thedirection of motion of the source; sampling at least one portion of eachsaid resolved discerned transient effects after the abruptdiscontinuities in the primary waveform; sampling at least one furtherportion of one individual component discerned transient effect; andseparately amplifying and recording at least a portion of said illsampled portions thereby to detect the presence of secondary transienteffects emanating from conducting bodies energized by said primarywaveform whereby the presence of said conducting bodies is indicated.

2. The method of detecting conducting bodies, and comprising: initiatingprimary waveforms exhibiting abrupt discontinuities at predeterminedintervals by means of a pulse of generally half sine wave shape;radiating said primary waveforms towards areas in which conductingbodies are to be detected thereby to induce secondary transient effectsemanating from conducting bodies energized by said primary waveforms;receiving said primary waveforms and said secondary transient effectsfiltering out extraneous received signals; sampling portions of saidsecondary transient effects at intervals locked in time relationship tothe predetermined abrupt discontinuities in the primary waveforms;passing said sampled portions through an amplification stage tuned tothe same frequency as the repetitionary of said abrupt discontinuitiesin the primary waveforms; further sampling portions of the amplifiedsignal; integrating said last mentioned waveforms into a fluctuatingdirect current; and utilizing said current to operate a recorderproducing a recording related to the received secondary transientsignals thereby indicating their presence.

3. The method of detecting conducting bodies as claimed in claim 2including the steps of triggering said abrupt discontinuities in theprimary waveforms and said sampled portions of the received signal inpredetermined time relationship.

4. The method of detecting conducting bodies and comprising: initiatingprimary electromagnetic waveforms from a source by means of a pulse ofbetween about 0.5 to 5 milliseconds duration, said waveforms exhibitingabrupt discontinuities at predetermined intervals thereby to inducetransient secondary electromagnetic waveforms emanating from saidconducting bodies delayed a short time after said abrupt discontinuitiesof said primary waveforms, receiving said primary waveforms; and saiddelayed transient secondary waveforms sampling at least one portion ofsaid delayed transient secondary waveforms at a predetermined timeinterval after said abrupt the presence of such sampled portions therebyto indicate discontinuities in said primary waveforms; and recording thepresence of conducting bodies.

5. The method of detecting conducting bodies and comprising: initiatingprimary electromagnetic waveforms from a source by means of a pulse ofgenerally half sine wave shape, said waveforms exhibiting abruptdiscontinuities at predetermined intervals thereby to induce transientsecondary electromagnetic Waveforms emanating from said conductingbodies delayed a short time after said abrupt discontinuities of saidprimary waveforms; receiving said primary waveforms and said delayedtransient secondary waveforms; discerning said delayed transientsecondary waveforms in three component directions mutually at rightangles thereby to provide three resolved components thereof; sampling atleast one portion of said resolved components at a predetermined timeinterval after said abrupt discontinuities in said primary waveforms;and recording the presence of such sampled portions thereby to indicatethe presence of conducting bodies.

6. The method of detecting conducting bodies as claimed in claim 5, inwhich one of the discerned components is arranged to be substantiallyhorizontal.

7. The method of detecting massive and disseminated ore deposits andother like electrically polarizable bodies and comprising: initiatingprimary electromagnetic waveforms from a source by means of a pulse ofbetween 0.5 to 5 milliseconds duration, said waveforms exhibiting abruptdiscontinuities at predetermined intervals thereby to induce transientpolarization in said body characterized by transient polarizationeffects emanating therefrom delayed a short time after said abruptdiscontinuities of said primary waveforms; receiving said primarywaveforms and said delayed transient effects; sampling at least oneportion of said delayed transient effects at predetermined intervalsafter said abrupt discontinuities in said primary waveforms, andrecording the presence of such sampled portions thereby to discriminatethe response of said polarizable body from adjacent non-polarizablebodties of similar conductivity.

8. The method of detecting massive and disseminated ore deposits andother like electrically polarizable bodies and comprising; initiatingprimary electromagnetic waveforms from a source, by means of a pulse ofbetween about 0.5 to 5 milliseconds duration, said waveforms exhibitingabrupt discontinuities at predetermined intervals thereby to inducetransient polarization in said body characterized by transientpolarization effects emanating therefrom delayed a short time after saidabrupt discontinuities of said primary waveforms; receiving said primarywaveform and said delayed transient effects; discerning said delayedtransient effects in three component directions mutually at right anglesthereby to provide three resolved components thereof; sampling at leastone portion of said resolved components at predetermined intervals aftersaid abrupt discontinuities in said primary waveforms, and recording thepresence of such sampled portions thereby to discriminate the responseof said polarizable body from adjacent non-polarizable bodies of similarconductivity.

9. Apparatus for the remote detection of conducting bodies comprising:signal generating means for radiating primary electromagnetic waveformsby means of a pulse of between about 0.5 to 5 milliseconds durationexhibiting abrupt discontinuities at predetermined intervals; receivingmeans for discerning secondary signals induced by said primary waveformsand reradiated by a conducting body together with transient componentsof said secondary signals occurring after said abrupt discontinuities insaid primary waveforms; adjustable electronic detection means normallyblocking said signals and operable intermittently to sample portions ofsaid transient components; amplifier means amplifying said sampledportions; second adjustable detector means operable to sample portionsof the amplified signal from said amplifier means; means integratingsaid last mentioned sampled portions; and means for recording saidintegrated signals, thereby to provide evidence of the presence ofconducting bodies.

10. Apparatus for the remote detection of conducting bodies comprising:signal generating means for radiating primary electromagnetic waveformsexhibiting abrupt discontinuities at predetermined intervals; inductivereceiving means discerning secondary signals induced by said primarywaveforms and reradiated by a conducting body and secondary transientcomponents of said secondary signals occurring after said abruptdiscontinuities in said primary waveforms; adjustable electronicdetection means normally blocking said signals and operableintermittently to sample portions of said discerned secondary transientcomponents during the absence of said primary waveform; amplifier meansamplifying said sampled portions of said discerned signals; integratormeans integrating said last mentioned sampled portions into a slowlyfluctuating direct current; and recording means for recording saidintegrated signals, thereby to provide evidence of the presence ofconducting bodies.

11. Apparatus for the remote detection of conducting bodies comprising:signal generating means for radiating primary electromagnetic waveformsexhibiting abrupt discontinuities at predetermined intervals; inductivereceiving means discerning secondary signals induced by said primarywaveforms and reradiated by a conducting body and secondary transientcomponents of said secondary signals occurring after said abruptdiscontinuities in said primary Waveforms; adjustable detection meansnormally blocking said signals and operable intermittently to sampleportions of said discerned secondary transient components following thesaid abrupt discontinuities in the primary waveform; amplifier meansamplifying said sampled portions of said discerned signals; secondadjustable detector means for sampling portions of the amplified signalfrom said last mentioned sampled portions; integrator means integratingsaid last mentioned sampled portions into a slowly fluctuating directcurrent; and recording means for recording said integrated signals,thereby to provide evidence of the presence of conducting bodies.

12. Apparatus for the remote detection of conducting bodies comprising:transmitter means generating a primary electromagnetic waveformexhibiting abrupt discontinuities at a predetermined frequency;inductive receiving means for discerning secondary signals induced bysaid primary waveforms and reradiated by a conducting body together withany transient components of said secondary signals occurring after saidabrupt discontinuities in said primary waveforms; amplification meansamplifying said discerned signals; adjustable means filtering saiddiscerned signals; adjustable detection means normally blocking saidsignals and operable intermittently to sample portions of said discernedsecondary transient components following said discontinuities in theprimary electromagnetic waveform; tuned amplifier means amplifying saidsampled portions of said discerned signals, said amplifier means beingtuned to the frequency of said abrupt discontinuities in the primarywaveform; second adjustable detector means for sampling portions of theamplified signal from said tuned amplifier means; means integrating saidlast mentioned sampled portions; and recording means for recording saidintegrated signals, thereby to provide graphic evidence of the presenceof conducting bodies.

13. Apparatus for the remote detection of conducting bodies comprising:a discontinuous waveform transmitter for transmitting primaryelectromagnetic waveforms exhibiting abrupt discontinuities at apredetermined frequency; at least one coil receiving means fordiscerning secondary signals induced by said primary waveforms andreradiated by a conducting body and transient components of said signalsoccurring after said abrupt discontinuities in said primary waveforms;pre-amplifier means amplifying said discerned signals; first detectorgate means, said first detector being adjustable to sample predeterminedportions of said pre-amplified discerned secondary transient components;tuned amplifier means amplifying said sampled portions, said lastmentioned amplifier means being tuned to the frequency of said abruptdiscontinuities in the primary waveform; second detector gate meansadjustable to sample predetermined portions of the signal from saidtuned amplifier means; high sensitivity recording means; and low passfilter integrator means converting the signals received from said seconddetector gate means to a slowly fluctuating DC. signal, said lastmentioned signal being utilized to energize said recorder thereby toform a record of varying form said form being related to the secondarytransient components induced by the energization of a conducting body.

14. Apparatus for the detection of conducting bodies as claimed in claim13 in which is provided at least two coil receiving means having theiraxes disposed mutually at right angles, the signals discerned by saidcoils being recorded by means of separate circuits.

15. Apparatus for the detection of conducting bodies as claimed in claim14 in which said first detector gate means comprises at least twoseparate detector gates sampling portions of a transient secondarycomponent at predetermined intervals.

16. Apparatus for the remote detection of conducting bodies comprising:a discontinuous waveform transmitter for transmitting primaryelectromagnetic waveforms exhibiting abrupt discontinuities at apredetermined frequency; a vertical transverse receiving coil; avertical longitudinal receiving coil; a horizontal receiving coil,

said receiving coils being arranged with their axes mu tuallyperpendicular respectively to discern components of signals reradiatedby a conducting body together with secondary transient signals occurringafter said abrupt discontinuities in said primary waveforms;pre-amplifier means amplifying said discerned signals in said receivercoils; first detector gate means, each said means sampling apredetermined portion of said pre-amplified discerned secondarytransient signals; tuned amplifier means separately amplifying each saidsampled portion, said last mentioned amplifier means being tuned to thefrequency repetition of said abrupt discontinuities in the primarywaveform; separate second detector gate means sampling the peak portionsof the signals from said second detector gate means; high sensitivityrecording means; and low pass filter integrator means separatelyconverting the signals received from each said second detector means toa slowly fluctuating direct current signal, each said last mentionedsignals being utilized separately to actuate said recording meansthereby to provide a record of varying form said form being related tothe sampled portion of the secondary transient signals discerned in saidcoils.

17. Apparatus for the remote detection of conducting bodies as claimedin claim 16 wherein said first detector gate means are provided tosample portions of said discerned secondary transient signalsimmediately following said abrupt discontinuities in the primarywaveform, and further first detector gate means are provided to sampleat least one other portion of said discerned secondary transient signalsfor at least one of said coils.

18. Apparatus for the remote detection of conducting bodies as claimedin claim 17 wherein there is provided triggering means activating saidfirst and said second detector gate means in adjustably predeterminedtime relationship to said abrupt discontinuities in the primaryelectromagnetic waveform.

19. The method of detecting conducting bodies as claimed in claim 1wherein said primary electromagnetic waveforms are initiated by means ofa pulse of generally half sine wave shape.

20. The method of detecting conducting bodies as claimed in claim 1wherein said primary electromagnetic waveforms are initiated by means ofa pulse of generally saw tooth shape.

21. The method of detecting conducting bodies as claimed in claim Iwherein said primary electromagnetic waveforms are initiated by means ofa pulse of generally square wave shape.

22. The method of detecting conducting bodies as claimed in claim 4wherein said primary electromagnetic waveforms are initiated by means ofa pulse of generally half sine wave shape.

23. The method of detecting conducting bodies as claimed in claim 4wherein said primary electromagnetic waveforms are initiated by means ofa pulse of generally saw tooth shape.

24. The method of detecting conducting bodies as claimed in claim 4wherein said primary electromagnetic waveforms are initiated by means ofa pulse of generally square wave shape.

25. The method of detecting massive and disseminated ore deposits andother like electrically polarizable bodies as claimed in claim 7 whereinsaid primary electromagnettic waveforms are initiated by means of apulse of generally half sine wave shape.

26. The method of detecting massive and disseminated ore deposits andother like electrically polarizable bodies as claimed in claim 7 whereinsaid primary electromagnetic waveforms are initiated by means of a pulseof generally saw tooth shape.

27. The method of detecting massive and disseminated ore deposits andother like electrically polarizable bodies as claimed in claim 7 whereinsaid primary electromagnetic waveforms are initiated by means of a pulseof generally square wave shape.

28. The method as claimed in claim 1 wherein said pulse is of between 1and 3 milliseconds duration.

29. The method as claimed in claim 4 wherein said pulse is of between 1and 3 milliseconds duration.

30. The method as claimed in claim 7 wherein said pulse is of between 1and 3 milliseconds duration.

31. The method as claimed in claim 8 wherein said pulse is of between 1and 3 milliseconds duration.

32. The method as claimed in claim 9 wherein said pulse is of between 1and 3 milliseconds duration.

35'. The method as claimed in claim 2 wherein said primary waveforms areinitiated from a moving source and wherein said primary waveform andsaid secondary transient efiects are received by means moving at avelocity substantially equal to said source.

34. The method as claimed in claim 4 wherein said primary waveforms areinitiated from a moving source and wherein said primary waveform andsaid secondary transient efiects are received by means moving at a velocity substantially equal to said source.

35. The method as claimed in claim 7 wherein said primary waveforms areinitiated from a moving source and wherein said primary waveform andsaid secondary transient efiects are received by means moving at avelocity substantially equal to said source.

36. The method as claimed in claim 8 wherein said primary waveforms areinitiated front a moving source and wherein said primary waveform andsaid secondary transient effects are received by means moving at avelocity substantially equal to said source.

37. The method of detecting conducting bodies and comprising: initiatingprimary waveforms exhibiting abrupt discontinuities at predeterminedintervals by means of a pulse of generally saw tooth shape; radiatingsaid primary waveforms towards said areas in which conducting bodies areto be detected thereby to induce secondary transient effects emanatingfrom conducting bodies energized by said primary waveforms; receivingsaid primary waveforms and said secondary transient eflects; filteringout extraneous received signals; sampling portions of said secondarytransient effects at intervals locked in time relationship to thepredetermined abrupt discontinuities in the primary waveforms; passingsaid sampled portions through an amplification stage tuned to the samefrequency as the repetitionary of said abrupt discontinuities in theprimary waveforms; further sampling portions of the amplified signal;integrating said last mentioned waveforms into a fluctuating directcurrent; and utilizing said current to operate a recorder producing arecording related to the received secondary transient signals therebyindicating their presence.

38. The method of detecting conducting bodies and comprising: initiatingprimary waveforms exhibiting abrupt discontinuities at predeterminedintervals by means of a pulse of generally square wave shape; radiatingsaid primary waveforms towards said areas in which conducting bodies areto be detected thereby to induce secondary transient efiects emanatingfrom conducting bodies energized by said primary waveforms; receivingsaid primary waveforms and said secondary transient efiects; filteringout extraneous received signals; sampling portions of said secondarytransient efiects at intervals locked in time relationship to thepredetermined abrupt discontinuities in the prhnary waveforms; passingsaid sampled portions through an amplification stage tuned to the samefrequency as the repetitionary of said abrupt discontinuities in theprimary waveforms; further sampling portions of the amplified signal;integrating said last mentioned waveforms into a fluctuating directcurrent; and utilizing said current to operate a recorder producing arecording relating to the received secondary transient signals therebyindicating their presence.

39. The method of detecting massive and disseminated ore deposits andother like electrically polarizable bodies and comprising. initiatingprimary electromagnetic waveforms from a source by means of a pulse ofgenerally half sine wave shape thereby to induce transient polarizationin said deposits and bodies characterized by transient polarizationefi'ects emanating therefrom delayed a short time after said abruptdiscontinuities of said primary waveforms; receiving said primarywaveforms and said transient polarization efiects; filtering outextraneous received signals; sampling portions of said transientpolarization efiects at intervals locked in time relationship to thepredetermined abrupt discontinuities in the primary waveforms; passingsaid sampled portions through an amplification stage tuned to the samefrequency as the repetitionary of said abrupt discontinuities in theprimary waveforms; further sampling portions of the amplified signals;integrating said last mentioned waveforms into a fluctuating directcurrent; and utilizing said current to operate a recorder producing arecording related to the received transient polarization efiects therebyindicating their presence.

40. The method of detecting massive and disseminated ore deposits andother like electrically polarizable bodies and comprising: initiatingprimary electromagnetic waveforms from a source by means of a pulse ofgenerally saw tooth shape thereby to induce transient polarization insaid deposits and bodies characterized by transient polarization efiectsemanating therefrom delayed a short time after said abruptdiscontinuities of said primary waveforms; receiving said primarywaveforms and said transient polarization eflects; filtering outextraneous received signals; sampling portions of said transientpolarization effects at intervals locked in time relationship to thepredetermined abrupt discontinuities in the primary waveforms; passingsaid sampled portions through an amplification stage tuned to the samefrequency as the repetitionary of said abrupt discontinuities in theprimary waveforms; further sampling portions of the amplified signals;integrating said last mentioned waveforms into a fluctuating directcurrent; and utilizing said current to operate a recorder producing arecording related to the received transient polarization efiects therebyindicating their presence.

41. The method of detecting massive and disseminated ore deposits andother like electrically polarizable bodies and comprising: initiatingprimary electromagnetic waveforms from a source by means of a pulse ofgenerally square wave shape thereby to induce transient polarization insaid deposits and bodies characterized by transient polarization eflectsemanating therefrom delayed a short time after said abruptdiscontinuities of said primary waveforms; receiving said primarywaveforms and said transsient polarization efiects, filtering outextraneous received signals; sampling portions of said transientpolarization efiects at intervals locked in time relationship to thepredetermined abrupt discontinuities in the primary waveforms; passingsaid sampled portions through an amplification stage tuned to the samefrequency as the repetitionary of said abrupt discontinuities in theprimary waveforms; further sampling portions of the amplified signals;integrating said last mentioned waveforms into a fluctuating directcurrent; and utilizing said current to operate a recorder producing arecording related to the received transient polarization efiects therebyindicating their presence.

42. The method as claimed in claim 2 wherein said abrupt discontinuitiesoccur at a repetition rate of between about 40 per second and 500 persecond.

43. The method as claimed in claim 4 wherein said abrupt discontinuitiesoccur at a repetition rate of between about 40 per second and 500 persecond.

44. The method as claimed in claim 5 wherein said abrupt discontinuitiesoccur at a repetition rate of between about 40 per second and 500 persecond.

45. The method as claimed in claim 39 wherein said abruptdiscontinuities occur at a repetition rate of between about 40 persecond and 500 per second.

46. The method as claimed in claim 40 wherein said abruptdiscontinuities occur at a repetition rate of between about 40 persecond and 500 per second.

47. The method as claimed in claim 41 wherein said abruptdiscontinuities occur at a repetition rate of between about 40 persecond and 500 per second.

48. The method as claimed in claim 7 wherein said abrupt discontinuitiesoccur at a repetition rate of between about 40 per second and 500 persecond.

References Cited by the Examiner The following references, cited by theExaminer, are of record in the patented file of this patent or theoriginal patent.

Bell System Technical Journal, vol. 27, 1948, pp. 26, 27.

OTHER REFERENCES WALTER L. CARLSON, Primary Examiner. GERARD R.STRECKER, Assistant Examiner.

